The goals of this project are to investigate the role of neu activation in mammary carcinogenesis and to identify genes overexpressed in neu-tumors that may be useful as prognostic markers or therapeutic targets for DCIS and invasive breast cancer. For these purposes, we will use a unique gene-transfer model system we recently developed. The first aim will clarify an important controversy. surrounding the mechanism of carcinogenesis by the activated (mutated) neu oncogene. We will ask if the mutated neu can transform mammary cells by a one-step mechanism. We will directly quantitate the probability of a mammary cell that expresses mutated neu of progressing to DCIS and invasive cancer. The second aim will extend our observations that mutated neu can completely transform ductal mammary cells by asking if activation of neu by overexpression, as found in human breast cancer, either completely transforms mammary cells or act as either an initiation event or an event capable of promoting a previously initiated mammary cells. In these studies, wild-type neu will be subcloned into an overexpression retrovirus vector. This vector will then be stably incorporated into in situ rat mammary cells. In some cases, these cells will be subject to hormonal promotion. Alternatively, this vector will be incorporated into a population of radiation-initiated cells. All infused glands will be followed for both neu associated DCIS and invasive cancers. The last aim will extend the molecular characterization of activated neu-associated mammary cancer and carcinogenesis by identifying genes that are either under- or overexpressed in this cancer. The proposed studies will provide good in vivo models of neu associated breast cancer. These will be used to delineate the role of neu activation in the etiology and. progression of both DCIS and invasive cancer. In addition, new prognostic markers and therapeutic targets will be sought for neu associated mammary cancer.